Photo-luminescence properties of Cu and Ag doped Li2B4O7 single crystals at low temperatures

UV excited photo luminescence from Li₂B₄O₇:Cu and Li₂B₄O₇:Cu, Ag single crystals has been investigated in the temperature range from 77 K to 300 K. An excitation band having a doublet structure at 240 nm and 262 nm was observed for the emission at 370 nm that corresponds to ¹A_1g→¹E_g and ¹A_1g→¹T_2g crystal field components of the 3d¹⁰→3d⁹4s¹ transition of Cu⁺. The relative intensity of these components and their temperature dependence provide a measure of the off-center displacement of the Cu⁺ ground state in the crystal lattice site. The co-doped Ag plays a role of a sensitizer when doped with Cu and increases the overall emission as the emission between Ag states lies in the excitation region of Cu states. The 370 nm emission in both the crystals slightly decreases with temperature; however a sudden increase in the intensity around 264 K was observed.     

Critical view on TL/OSL properties of Li2B4O7 nanoparticles doped with Cu,Ag and co-doping Cu,Ag: Dose response study

Small size (25 nm) Li₂B₄O₇ nanoparticles doped with different concentrations of Cu, Ag and co-doped with Cu, Ag were prepared by solid state sintering at 700 °C. The crystalline phase and particle sizes analysis were carried out by XRD and TEM. FTIR study reveals the formation of vibrational bonds at 1600–1200 cm⁻¹, 1500–700 cm⁻¹, 950–870 cm⁻¹ and 870–415 cm⁻¹. The kinetic parameters of the TL glow curves were evaluated using CGCD procedure in R-software. The CW-OSL decay curves were fitted with third order exponential decay curves and photoionization cross sections of each component were evaluated. The lifetime of the main TL dosimetric peak were also calculated to check the stability of the signal. Dose responses of the synthesized Li₂B₄O₇ nanoparticles for both the TL and CW-OSL were studied in the range of 0.02 mGy to50 Gy and found to be linear upto this range. Fading of the CW-OSL decay curves were also studied. The MDD of the synthesized samples were also calculated and observed to be 15 μGy.     

Thermoluminescence studies of thermally treated CaB4O7:Dy

Borate based thermoluminescence dosimeters (TLD) show high sensitivity and good TL characteristics. One of the promising material amongst the dosimeters is Dy doped CaB₄O₇. Spectrally resolved thermoluminescence of Dy doped CaB₄O₇ shows three glow peaks at about 50 °C, 240 °C and 380 °C, the intensity of the 240 °C glow peak being the maximum. All TL experiments were conducted on a high sensitivity TL spectrometer at Sussex University with a heating rate of 50 °C min^?1. Two main emissions associated with the Dy dopant are observed at ～480 and 580 nm. The samples were subjected to a series of treatments including excitation by X-rays and UV laser radiation. As part of the present research CaB₄O₇:Dy materials were subjected to two different heat treatments; quenching and slow cooling in order to investigate the changes in TL characteristics.     

Luminescence and defect studies of YAlO3:Dy3+, Sm3+ single crystals exposed to 100 MeV Si7+ ion beam

Ionoluminescence (IL) and photoluminescence (PL) spectra for different rare earth ions (Sm³⁺ and Dy³⁺) activated YAlO₃ single crystals have been induced with 100 MeV Si⁷⁺ ions with fluence of 7.81×10¹² ions cm^?2. Prominent IL and PL emission peaks in the range 550–725 nm in Sm³⁺ and 482–574 nm in Dy³⁺ were recorded. Variation of IL intensity in Dy³⁺ doped YAlO₃ single crystals was studied in the fluence range 7.81×10¹²–11.71×10¹² ions cm^?2. IL intensity is found to be high in lower ion fluences and it decreases with increase in ion fluence due to thermal quenching as a result of an increase in the sample temperature caused by ion beam irradiation. Thermoluminescence (TL) spectra were recorded for fluence of 5.2×10¹² ions cm^?2 on pure and doped crystals at a warming rate of 5 °C s^?1 at room temperature. Pure crystals show two glow peaks at 232 (Tg₁) and 328 °C (Tg₂). However, in Sm³⁺ doped crystals three glow peaks at 278 (Tg₁), 332 (Tg₂) and 384 °C (Tg₃) and two glow peaks at 278 (Tg₁) and 331 °C (Tg₂) in Dy³⁺ was recorded. The kinetic parameters (E, b s) were estimated using glow peak shape method. The decay of IL intensity was explained by excitation spike model.     

Luminescence and evaluation of trapping parameters in NaMgSO4Cl: X (X=Cu or Ce) phosphor

Thermoluminescence (TL) characteristics of recently developed high sensitive mixed halosulphate phosphors, NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce were studied in comparison with CaSO₄: Dy in order to assess the possibility of their use in personal monitoring and TLD phosphors at very low dose of 5 Gy. It was found that NaMgSO₄Cl: Cu is 5.59 times and NaMgSO₄Cl: Ce is 6.18 times more sensitive as compared to standard CaSO₄: Dy. UV photo-excited luminescence from Cu to Ce doped NaMgSO₄Cl halosulphate phosphors has been investigated. The intense emission of the spectrum is assigned to electronic transitions 3d⁹4s¹→3d¹⁰ in monovalent copper ion and 5d→4f in Ce³⁺ ions. Increase in PL peak intensity suggesting that Cu and Ce play an important role in PL emission in the present matrix. These phosphors were synthesized by the wet chemical method. XRD, photoluminescence (PL) and thermoluminescence (TL) characterization of phosphors has been reported in this paper. The preparation of an inexpensive and high sensitive NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce with TL glow peaks for different concentrations are observed between 160 and 195 °C and between 200 and 225 °C, respectively, exposed to gamma-rays of ⁶⁰Co for their thermoluminescence (TL) properties. The glow curves have been recorded at a heating rate of 2 K s^?1 and irradiated at a dose rate of 0.995 kGy h^?1 for 5 Gy. In present study the trapping parameters such as order of kinetics (b), activation energy (E) and frequency factors (s) have been calculated for the 195 and 200 °C glow peaks of NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce, respectively by using Chen's method. The paper discusses the luminescence of Cu⁺ and Ce³⁺ by simple method of incorporation in NaMgSO₄Cl host.     

Thermoluminescence studies on Cu-doped Li2B4O7 single crystals

Lithium tetraborate (Li₂B₄O₇) is a tissue equivalent material and single crystals of this material doped with Cu are promising for dosimetric applications. In the present study highly transparent single crystals of lithium tetraborate (Li₂B₄O₇) doped with Cu (0.5 wt%) have been grown using the Czochralski technique. The Li₂B₄O₇:Cu crystals were studied using photoluminescence, X-ray diffraction (XRD), UV-vis transmission, time resolved fluorescence and thermoluminescence (TL) techniques. The TL readout of Li₂B₄O₇:Cu crystals showed two well-defined glow peaks at 402 K (peak-1) and 513 K (peak-2) for a 4 K/s heating rate. While the low temperature TL peak-1 fades completely within 24 h at room temperatures, the main dosimetric peak-2 remains the same. The TL sensitivity of the grown single crystal is found to be 3.3 times that of a conventional TL phosphor, TLD-100. The Li₂B₄O₇:Cu crystals showed a linear TL dose-response in the range from 1 mGy to 1 kGy. The TL analysis using a variable dose method revealed first order kinetics for both the peaks. Trap depth and frequency factor for peak-1 were found to be 0.81 eV and 5.2×10⁹ s⁻¹, whereas for peak-2 the values were 1.7 eV and 1.7×10¹⁶ s⁻¹, respectively.     

Effect of Eu and Pb doping on the dosimetric properties of LiCAF

LiCaAlF₆ (LiCAF) crystals doped with two different ions (europium and lead) have been investigated as potential new dosimetric materials. The stability of thermally stimulated luminescence (TSL) glow peaks in LiCAF:Eu was evaluated by means of the initial rise technique. The decay times at room temperature of the traps related to the dosimetric glow peaks were found to range between 40 and 2 × 10⁴ years confirming the good dosimetric characteristics of this crystal. The glow curve of LiCAF:Pb is dominated by a peak at approximately 300 °C emitting in the UV region (³P_0,1–¹S₀ transition of Pb²⁺) superimposed to a very broad structure at lower temperature (20–200 °C) featuring recombination at an intrinsic defect centre. The anomalous behavior of the low temperature structure during thermal cleaning procedures prevented any reliable numerical analysis of the TSL glow peak at 300 °C.     

Thermoluminescence kinetics of lithium borate crystals

The paper reports the results of experimental study and modeling of thermally stimulated recombination processes in undoped crystals of lithium borates Li₂B₄O₇ and LiB₃O₅. For both crystals in the unified experimental conditions, we have measured the glow curves of thermally stimulated luminescence, the spectra and the temperature dependencies of intensity of the steady-state luminescence over the temperature range of 90–500 K under X-rays excitation. In the framework of the unified model, we carried out the calculations of the recombination processes and interpretation of the results with the involvement of all the available experimental data on undoped lithium borate crystals.     

Lithium-ionic diffusion and electrical conduction in the Li7La3Ta2O13 compounds

The electrical properties and the mechanism of lithium ionic diffusion in the Li₇La₃Ta₂O₁₃ compounds were investigated. The bulk and total conductivity at 300 K of the Li₇La₃Ta₂O₁₃ compound are about 3.3 × 10^? 6 S/cm and 2.6 × 10^? 6 S/cm, respectively. The activation energy of bulk and total conductivity is in the range of 0.38–0.4 eV. A prominent internal friction peak in Li₇La₃Ta₂O₁₃ compounds was observed around 280 K at 0.5 Hz, which is actually composed of two subpeaks (P₁ peak at lower temperature and P₂ peak at higher temperature). From the shift of peak position with frequency, the activation energy of 1.0 eV and the pre-exponential factor of relaxation time in the order of 10^? 18–10^? 21 s were obtained if one assumes Debye relaxation processes. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Based on the coupling model, the relaxation activation energies are deduced as 0.45 eV and the pre-exponential factor of relaxation time as 10^? 15 s. Judging from these relaxation parameters and the similarity of structure between Li₇La₃Ta₂O₁₃ and Li₅La₃Ta₂O₁₂ compounds, the P₁ and P₂ peaks are suggested to be related with the lithium ionic diffusion between 48g?48g and 24d?48g.     

Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li₆CaLa₂Ta₂O₁₂ and a = 13.001(4) Å for Li₆BaLa₂Ta₂O₁₂. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li⁺ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ samples exhibit the conductivity σ = 2.2 × 10^? 6 S cm^? 1 at 27 °C (E_a = 0.50 eV) and σ = 1.3 × 10^? 5 S cm^? 1 at 25 °C (E_a = 0.44 eV), respectively.     

Optimization of CW-OSL parameters for improved dose detection threshold in Al2O3:C

Continuous wave optically stimulated luminescence (CW-OSL) is relatively a simple technique that offers good signal to noise ratio (SNR) and involves simple instrumentation. This study reports the influence and optimization of CW-OSL parameters on minimum detectable dose (MDD) using α-Al₂O₃:C phosphor. It is found that at a given stimulation intensity MDD in CW-OSL mode depends on signal integration time. At lower integration times MDD is inferior. It exhibits an improvement for intermediate values, shows a plateau region and deteriorates as integration time increases further. MDD is found to be ∼127 μGy at 4 mW/cm² stimulation intensity for integration time of 0.1 s, which improves to ∼10.5 μGy for 60 s. At stimulation intensity of 72 mW/cm², MDD is 37 μGy for integration time of 60 s and improves significantly to 7 μGy for 1 s.     

Mechanoluminescence,thermoluminescence and photoluminescence studies on Al2O3:Tb phosphors

Terbium activated Al₂O₃ phosphors were synthesized by combustion technique using hydrazine as a reductive non-carbonaceous fuel. X-ray diffraction (XRD) patterns of the samples were recorded to confirm the formation of the sample. Scanning electron microscope (SEM) images were taken to study the surface morphology of the sample. The photoluminescence (PL), thermoluminescence (TL) and mechanoluminescence (ML) properties of the γ-ray irradiated samples were studied. ML was excited impulsively by dropping a piston on the sample. In ML glow curves one peak with a shoulder was observed. ML intensity increases with activator concentration. Optimum ML was observed for the sample having 0.5 mol% of Tb ions. In the TL glow curve two distinct peaks, one around 222 °C and another around 280 °C, were observed for the samples having 0.5 mol% of activator concentration. In the PL spectra the ⁵D₄→⁷F₅ line at 544 nm in the green region is observed, which is the strongest in Al₂O₃ system. It is suggested that de-trapping of trapped charge carriers followed by recombination is responsible for ML and TL in this system.     

Thermo,Iono and photoluminescence properties of 100 MeV Si7+ ions bombarded CaSiO3:Eu3+ nanophosphor

This paper reports on the thermo (TL), iono (IL) and photoluminescence (PL) properties of nanocrystalline CaSiO₃:Eu³⁺ (1–5 mol %) bombarded with 100 MeV Si⁷⁺ ions for the first time. The effect of different dopant concentrations and influence of ion fluence has been discussed. The characteristic emission peaks ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) of Eu³⁺ ions was recorded in both PL (1×10¹¹–1×10¹³ ions cm^?2) and IL (4.16×10¹²–6.77×10¹² ions cm^?2) spectra. It is observed that PL intensity increases with ion fluence, whereas in IL the peaks intensity increases up to fluence 5.20×10¹² ions cm^?2, then it decreases. A well resolved TL glow peak at ～304 °C was recorded in all the ion bombarded samples at a warming rate of 5 °C s^?1. The TL intensity is found to be maximum at 5 mol% Eu³⁺ concentration. Further, TL intensity increases sub linearly with shifting of glow peak towards lower temperature with ion fluence.     

Effect of doping on the luminescence properties of Li2B4O7

In this work, we have investigated the influence of doping agents on the luminescence properties of multiply doped Li₂B₄O₇ and the temperature lag between TSL materials and the heating element. The results of thermoluminescence studies show that the Ag doping leads to the appearance of a new glow curve peak at 165 °C and the increasing sensitivity of Li₂B₄O₇:Cu,Ag,P is correlated with copper and phosphate concentrations. Under the excitation at 245 nm the emission spectra show maxima at 365 and 450 nm in the ceramic, crystal and glass. The low energy shift in the latter system might be related to the local structural distortion in the glass around Cu⁺ ions.     

5D0→7F1 and 5D0→7F2 transition in europium doped halosulphates for mercury-free lamps

Polycrystalline Na₃SO₄F:Eu and NaMgSO₄F:Eu halosulphate phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics. Two well resolved peaks are observed at 593 nm and 614 nm, which are assigned to due to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions. TL is observed at temperatures between 100 °C and 300 °C. In this paper, we report PL emission spectra of Eu³⁺ and TL glow curves, which are more sensitive than the standard TLD-CaSO₄:Dy. The presented phosphors are applicable for the mercury free lamps and solid state lighting devices.     

Studies on shallow traps in Li2B4O7:Eu,Mn

Li₂B₄O₇ (LTB) single crystals doped with 0.5 mol% Mn and 0.005 mol% Eu have been grown by the Czochralski method. The presence of Eu³⁺ has been confirmed by photoluminescence spectra of non-irradiated crystals, whereas the presence of Mn²⁺ by absorption spectra of gamma-irradiated ones, as well as by EPR measurements. Unlike in most thermoluminescence studies on pure and doped LTB, performed usually above 300 K, glow curves have been recorded between 10 and 300 K in order to focus the attention on shallow traps. A broad, intense glow peak is observed around 80 K, with three weaker peaks at 205, 255, and 280 K. Based on supplementary T_max − T_stop experiments, the trap parameters have been derived assuming that the glow curve is in fact formed by a superposition of a double Gaussian band related to a quasi-continuous distribution of trapping levels, and several glow peaks produced by discrete traps. The nature of the traps is also discussed.     

Synthesis and electrochemical properties of Li2S–B2S3–Li4SiO4

New Li⁺ ion-conductive glasses Li₂S–B₂S₃–Li₄SiO₄ were synthesized by rapid quenching, and they were transformed into glass ceramics by heat treatment. The heat treatment increased the ionic conductivities of the Li₄SiO₄-doped glasses, and the highest ionic conductivity observed in the system was 1.0 × 10^− 3 S cm^− 1 at room temperature. The glass ceramics were highly stable against electrochemical oxidation with a wide electrochemical window of 10 V.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Investigation of thermoluminescence in Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> phosphors doped with Cu,Ag and Mg

Li2B4O7 (LBO)Cu,Ag,Mg phosphors have been prepared by the sintering technique.The roles of the Ag and Mg dopants in the phosphors have been studied using the methods of thermoluminescence (TL) glow curves and TL 3D spectra. The results indicated that proper concentrations of Ag and Mg can enhance the TL of LBOCu.It was also indicated that the intensity of TL peak at ～130℃ is reduced with the increasing Ag concentration, and enhanced with the increasing Mg concentration.From the TL 3D spectra, three emission bands (λ1 = 421 nm,λ2 = 380 nm, λ3 = 350nm) were observed the intensity of low energy emission band is reduced and that of the high energy is enhanced with the increasing dopant Ag; on the contrary, the intensity of low energy emission band is enhanced and that of the high energy one is reduced with the increasing dopant Mg.     

Li4Ti5O12/Ag composite as electrode materials for lithium-ion battery

The Li₄Ti₅O₁₂/Ag composites were prepared by thermal decomposition of AgNO₃ added to Li₄Ti₅O₁₂ powders. The influence of the Ag contents and the mixing media on the particle size, morphology and electrochemical performance of Li₄Ti₅O₁₂/Ag composites were investigated. The highest discharge capacity of the Li₄Ti₅O₁₂/Ag composite reached at the 5 wt.% of Ag content. Compared with alcohol medium, distilled water as mixing medium presented the Li₄Ti₅O₁₂/Ag composite with higher specific capacity and better cycling performance, leading to a reversible capacity after 50 cycles of 184.2 mAh/g with a capacity degradation of 3.31% compared to the second cycle at 2 C rate.